A. Field of the Invention
This invention relates to the field of normally closed solenoid operated valve assemblies.
B. Prior Art
It has been known in the prior art to use relief valves which are solely mechanically operated. By varying the preload on a spring in the relief valve, there is adjusted the amount by which the fluid would have to increase to open the valve. When the fluid pressure reaches a predetermined value, the valve opens and allows the fluid pressure to be relieved. In this manner, the valve is able to maintain a predetermined pressure. Such mechanically operated relief valves only maintain pressure at a predetermined value if the pressure is below that value but have no effect on fluid flow.
For high fluid volumes, it has been known to use such mechanical relief valves in combination with a second stage in which upstream pressure is applied under the poppet of the second stage. Upstream pressure is divided and applied above the poppet as an intermediate pressure. The mechanical relief valve is connected between tank and above the second stage poppet. Accordingly, when pressure above the poppet exceeds a predetermined value set by the mechanical relief valve, the second stage opens and allows upstream pressure to flow to tank. In addition, it has been known to remotely electrically dump the system by means of a normally closed solenoid valve connected above the second stage poppet. When actuated, the solenoid valve dumps intermediate pressure to thereby allow upstream pressure to flow to tank. However, this combination of a separate mechanical relief valve and a separate solenoid valve has left much to be desired since use of these two is not only cumbersome but also quite costly.